1. Field of the Invention
The present invention relates to a liquid crystal display device and more specifically to a liquid crystal display device to realize soft touch and sense touch regardless of illumination of external light or reflected light.
2. Discussion of the Related Art
In accordance with the recent information-dependent society, displays to visualize electric information signals have been rapidly developed. Various slim, lightweight, and low-power flat display devices are commonly used as alternatives to conventional cathode ray tubes (CRTs).
Examples of flat display devices include liquid crystal display devices (LCDs), plasma display panel devices (PDPs), field emission display devices (FEDs), electro luminescence display devices (ELDs) and the like. These flat display devices necessarily require a flat display panel to realize an image wherein the flat display panel has a structure in which a pair of transparent insulating substrates is joined such that an inherent luminous or polarized material layer is interposed between the substrates. Among flat display devices, liquid crystal display devices control light transmittance of liquid crystals using an electric field to display an image. For this purpose, an image display device includes a display panel including liquid crystal cells, a backlight unit to irradiate light to the display panel and an operation circuit to operate liquid crystal cells.
The display panel is formed such that a plurality of gate lines cross a plurality of data lines to define a plurality of unit pixel regions. Each pixel region includes a thin film transistor array substrate and a color filter array substrate facing each other, a spacer interposed between the two substrates to maintain a predetermined cell gap, and a liquid crystal filled in the cell gap.
The thin film transistor array substrate includes a plurality of gate lines and a plurality of data lines, a thin film transistor formed as a switching device at each intersection between the gate line and the data line, a pixel electrode arranged in each liquid crystal cell and connected to the thin film transistor, and an orientation film coated on the resulting structure. The gate lines and data lines receive signals through respective pad portions from operation circuits.
In response to scan signals supplied to the gate lines, the thin film transistor transfers pixel voltage signals supplied to data lines to the pixel electrode.
In addition, the color filter array substrate includes color filters arranged in respective liquid crystal cells, a black matrix to partition the color filters and reflect external light, a common electrode to supply a reference voltage to the liquid crystal cells, and an orientation film coated on the resulting structure.
The thin film transistor substrate and the color filter array substrate thus separately formed are arranged and then joined such that the two substrates face each other, liquid crystal is injected into a region provided between the substrates, and the region is sealed, to complete fabrication of the liquid crystal display device.
Meanwhile, the liquid crystal display device is a positive light-emitting device, which controls luminance of screen using light from the backlight unit arranged on the back surface of the liquid crystal panel.
Recently, technologies for adhering a touch screen panel onto such a liquid crystal display device were suggested. The touch screen panel refers to a user interface which is generally adhered to the display device and undergoes variations in electric properties thereof at a touch position contacting a non-transparent material such as a finger or a pen to sense the touch position. Touch screen panel-adhered liquid crystal display devices detect information of a position at which a finger of user or a touch pen contacts a screen, and realize a variety of applications based on the detected information.
However, such a liquid crystal display device has various disadvantages of increased production costs due to the touch screen panel, decreased yield due to processes for adhering the touch screen panel to the liquid crystal panel, and deteriorated luminance and increased thickness of the liquid crystal panel.
In order to solve these disadvantages, a great deal of attempts have been made to form a photosensor inside a display panel, and thereby control the backlight unit according to the luminance of external light and solve the increased volume of the photosensor adhered onto the external surface of the display panel.
Hereinafter, a related art liquid crystal display device will be described with reference to the annexed drawings.
FIG. 1 is a schematic sectional view illustrating a general liquid crystal display device provided with a photo-sensing type touch panel.
As shown in FIG. 1, a related art liquid crystal display device provided with a photo-sensing type touch panel includes a liquid crystal panel 10 including a first substrate 1 and a second substrate 2 which face each other, a pixel transistor (pixel TFT) 3 arranged on the first substrate 1, a sensing transistor (sensor TFT) 4, and a backlight unit 20 arranged under the liquid crystal panel 10 to the liquid crystal panel 10 to transfer light to the liquid crystal panel 10.
In addition, the photo-sensing is realized through the following mechanism. When light is emitted from the backlight unit 20 through the liquid crystal panel 10 and the liquid crystal panel is then touched by a finger or another material 30, light to be emitted is reflected to the liquid crystal panel 10 again and a sensing transistor senses the light.
This related art liquid crystal display device provided with a photo-sensing type touch panel senses touch based on a reflection level of external light, thus greatly varying touch sensitivity depending on illumination of the external light. For example, when there is no or almost no difference between external light and shielded portion upon touch, touch sensation cannot be realized.
The related art liquid crystal display device provided with a touch panel has the following disadvantages.
First, the related art touch panel-integrated liquid crystal display device senses light reflected from a backlight upon touch as well as external light, thus allowing a sensor to fail to accurately sense input signals, when there is almost no difference between reflected light and external light.
Second, input signals upon touch are varied depending on external environment, thus complicating algorithms for position detection and causing high possibility of malfunction. That is, when illuminance of external light is higher than that of reflected light, signals upon touch are output at a voltage lower than that of the neighboring region, and on the other hand, when illuminance of external light is lower than that of reflected light, touch signals generate a voltage higher than that of the neighboring region, making it difficult to sense the actual touch signals. For example, when an external environment is under high illumination, although a touch material such as fingers does not touch the liquid crystal panel, shadows generated by the external light may be disadvantageously mistaken as actual touch.